The invention relates to a picture display device comprising a cathode ray tube which is provided with a deflection unit having deflection coils and a picture balance correction system with a potentiometer with which the picture balance error can be reduced. The invention also relates to a deflection unit and to a picture balance correction system.
A cathode ray tube is suitable as a pick-up tube or a display tube but may be alternatively applied in apparatus for Auger spectroscopy, electron microscopy and electron lithography.
A cathode ray tube for a monochrome display device, for example a television or monitor, has a glass envelope which is composed of a screen and a cone. The widest side of the cone is secured to the screen. Its narrowest side terminates in a tubular end having a substantially circular cross-section, which end is referred to as the neck. The screen is provided with a display screen consisting of a phosphor layer. The tubular end accommodates an electron gun which emits an electron beam during operation. This beam can be sent to a given location on the display screen by means of deflection coils which generate a given magnetic field.
The display screen is activated by scanning the electron beam alongside it, the beam being modulated by a video signal. This video signal ensures that the phosphor is excited in accordance with such a pattern that its luminescence produces an image. When many electrons land on a pixel during the excitation time of this pixel, the image lights up more brightly. The video signal is applied to the cathode via electric current conductors.
There are many pixels per surface unit. Moreover, the pixels are excited one after the other within a very short time. The viewer thereby experiences a moving image from a normal viewing distance.
In a color display device, for example a color television or a color monitor, each pixel consists of three phosphor elements each luminescing in a different primary color. As it were, there are three uniform regular patterns on the display screen, each pattern having a different luminescence color. Instead of one electron beam, three electron beams emitted by three different cathodes in the color electron gun are scanned along the screen during operation. Each of these three beams excites the pixels with a given luminescence color. Since the phosphor elements of a pixel are located close together, the viewer experiences them as a single element instead of separate elements. The color which is experienced is a mixed color of the three elements. By exciting each element with a given intensity, the viewer experiences a given color. For example, if the red element and the blue element are fully excited and the green element is minimally excited, the viewer will experience the mixed color of purple. Moreover, similarly as for a monochrome cathode ray tube, the pixels are located so close together that the viewer does not see them separately from a normal viewing distance. This produces a color image.
During the production of a picture display device, a deflection unit is placed on the cathode ray tube. This unit comprises a field deflection coil and a line deflection coil. With these coils, the deflection of the electrons emitted by the electron gun towards the desired location on the screen is ensured during operation of the picture display device. The field deflection coil deflects electrons in the vertical direction and the line deflection coil deflects electrons in the horizontal direction. When the picture display device leaves the production line, it is checked how the electron beams behave during operation. In most picture display devices, these beams do not initially land on the correct location on the display screen. To improve the landing, a number of different corrections can be applied. Different correction methods are suitable for each type of landing error. To reduce deviations in the vertical direction on the vertical axis, a potentiometer is used. Said type of deviation is referred to as picture balance error and the potentiometer is therefore referred to as picture balance potentiometer. The ratio between the currents through the two field deflection coil halves can be controlled by means of this potentiometer. In this way, the picture balance error can be corrected at zero. A conventional potentiometer comprises a resistor which is placed in a housing consisting of a metal cap and a synthetic material base. An insulation ring is arranged between the resistor and the housing. The housing also accommodates a shaft to which a wiper is secured which makes electrically conducting contact with the resistor and can be displaced. Dependent on the position where the wiper touches the resistor, it receives a different voltage. A contact ring leading the picked-up voltage to the exterior is present between the wiper and the base.
Usually, a picture balance potentiometer on a PCB (printed circuit board) is secured to the deflection unit. The PCB is provided with conducting patterns which extend towards pins to which the connections with the field deflection coil are sealed. Such a construction is shown in, for example FIG. 6, page 157 of Philips Technisch Tijdschrift (Philips Technical Review) vol. 39, no. 6/7. The deflection unit pictured at the bottom right in this Figure shows a PCB on the left-hand side with a potentiometer secured to it (in a cylindrical cap having a circular cross-section).
Said construction is, however, expensive and harmful to the environment. The use of a PCB is not without any risk either. If it were overheated, toxic substances would be released. Moreover, a PCB may easily break.